1. Field of the Invention
The present invention relates to an apparatus and method for limiting the leakage to ground current while optimizing output of a power supply adaptable for use with a motion sensor switch. More particularly, the present invention relates to an apparatus and method for employing in a power supply a regulator comprising a transistor and resistor arrangement which limits leakage to ground current of the power supply and optimizes output of the power supply for a range of input voltages.
2. Description of the Related Art
Motion sensor switches, such as Model 1WS-ZP-M and Model 1WS-ZP-277V motion sensor switches manufactured by Hubbell, Inc., include a motion sensor, such as a passive infrared detector (PIR), and an ambient light level sensor, such as a photocell. Other motion sensor switches are described in U.S. Pat. No. 5,821,642 to Nishihira et al., U.S. Pat. No. 5,699,243 to Eckel et al., and U.S. Pat. No. 4,874,962 to Hermans, the entire contents of each being incorporated herein by reference.
Motion sensor switches of this type can be used, for example, as an occupancy detector which shuts off lights in a room when the sensor detects that no one is present in the room, and turns on the lights in the room when a person enters the room. A motion sensor switch also can be used, for example, as a motion sensor for an alarm system.
During operation, the motion sensor of the motion sensor switch monitors an area, such as an office, conference room in a building, or a home, for the presence of movement. Specifically, the motion sensor detects a change in the infrared energy radiating from regions in the area monitored by different sensing lobes of the PIR detector, which generally have a pass band within the 8-14 .mu.m infrared range. If a person enters the monitored area, the person changes the amount of infrared energy being detected by the PIR detector. Therefore, the magnitude of the signal output by PIR detector, which is representative of the amount of detected infrared energy, will change. A motion sensor circuit in the motion sensor switch processes this signal, and provides a signal to a controller indicating that the amount of infrared energy received by the infrared detector has changed.
The controller interprets the signal provided by the motion sensor circuit, along with the signal provided by ambient light level sensor. If the signal provided by ambient light level sensor indicates that the ambient light in the monitored area is low (e.g., very little natural light is present in the monitored area), the control circuit will turn on or increase the brightness of the lights in the monitored area. However, if the signal provided by ambient light level sensor indicates that the ambient light in the monitored area is sufficient (e.g., due to sunlight, etc.), the control circuit may not turn on or brighten the lights, or may only brighten the lights slightly. In either event, control of the lights is based on the signals provided by the motion sensor and ambient light level sensor.
Motion sensor switches of this type also typically include a delay timing adjustment device, such as a potentiometer, which can be manually adjusted to set the delay time during which the lights should remain on after all occupants have left the monitored area. For example, if the delay timing adjustment device is adjusted to a 30-second setting, when all occupants leave and remain out of the monitored area for a period of time exceeding 30 seconds, the circuit of the motion sensor turns off the lighting load in the monitored area to conserve energy.
Motion sensor switches of the type described above are typically powered by a power supply circuit which receives an AC supply voltage, such as 110 VAC or 277 VAC, and converts the AC supply voltage into DC voltages of suitable levels, for example, +3 VDC, +6 VDC and +28 VDC for driving respective components of the motion sensor switch. Examples of power supply circuits are described in U.S. Pat. No. 4,874,962 to Hermans and U.S. Pat. No. 5,821,642 to Nishihira et al., cited above.
As described in these patents, the DC voltages are generated from a "leakage to ground current" that flows from the hot terminal of the AC supply voltage through, for example, a resistive element to earth ground. Specifically, the leakage to ground current generates an AC voltage across the resistive element, which is provided to a rectifier circuit, for example, that establishes a DC voltage of suitable magnitude (e.g., +28 VDC) at a power supply output terminal. Also, voltage divider circuits are coupled to the output terminal which further reduce the magnitude of the DC voltage to other desired magnitudes (e.g., +6 VDC and +3 VDC).
As further described in U.S. Pat. No. 4,874,962, in particular, U.L. Standards require that the leakage to ground current be limited to no greater than 500 .mu.A RMS in these types of power supplies which are used in commercial settings such as buildings and the like. To achieve this current limiting feature, the circuit described in U.S. Pat. No. 4,874,962 includes two resistors and a neon lamp coupled in series between the hot terminal of the AC supply voltage and the rectifier circuit that establishes the DC voltage.
On the other hand, the circuit described in U.S. Pat. No. 5,821,642 uses resistors having different magnitude resistance values depending on the magnitude of AC supply voltages of different magnitudes to limit the leakage to ground current in the power supply to less than 500 .mu.A RMS to comply with U.L. Standards. That is, for a 110 VAC supply voltage, the power supply uses a 226 k.OMEGA. resistor to limit the leakage to ground current to less than 500 .mu.A RMS. However, for a 277 VAC supply voltage, the power supply uses a 549 k.OMEGA. resistor to limit the leakage to ground current to less than 500 .mu.A RMS.
Although the types of resistive current regulators used in the power supplies described above are generally suitable to limit the leakage to ground current to comply with U.L Standards, those types of current limiters have certain drawbacks. In particular, the magnitude of current passing through these current regulators varies in proportion to a change in AC supply voltage. For example, if the AC supply voltage doubles, the magnitude of current passing through the current regulator also essentially doubles. Because this current is used to generate the DC output voltage of the power supply as described above, any fluctuation in current magnitude will cause a proportional fluctuation in the magnitude of the DC output voltage. Accordingly, the stability of the DC output voltage of power supplies including these resistive current regulators is largely dependent on the stability of the AC supply voltage.
Another type of regulator circuit 10 that can be used in the power supply circuits described above in place of the resistive regulator circuit is shown in FIG. 1. As illustrated, the regulator circuit 10 includes a plurality of NPN transistors 12 and 14 which are coupled to reduce fluctuations in leakage to ground current caused by fluctuations in the magnitude of the AC supply voltage, while also limiting the maximum value of the leakage to ground current to comply with U.L. Standards.
Specifically, in this arrangement, the collector of transistor 12 is coupled to the hot terminal of the AC supply voltage, and the base of this transistor 12 is coupled via a resistor 16 to the hot terminal. The emitter of this transistor 12 is coupled to the base of transistor 14, and is further coupled via a resistor 18 to the output terminal of the regulator circuit. The collector of transistor 14 is coupled to the base of the first transistor 102, and is further coupled via resistor 16 the to the hot terminal of the AC supply voltage. The emitter of transistor 14 is coupled to the output terminal of the regulator circuit.
The regulator circuit shown in FIG. 1 is more capable than the resistor regulator circuit of maintaining a stable leakage to ground current in response to a fluctuation in the magnitude of the AC supply voltage. However, this type of regulator circuit still allows for a significant fluctuation in the magnitude of the leakage current to ground in response to fluctuations in the AC supply voltage, as well as in response to changes in temperature. Hence, the stability of the DC output voltage of a power supply circuit including this type of current regulator is still largely dependent on the stability of the AC supply voltage.
Accordingly, a continuing need exists for a regulator for a DC power supply that limits the leakage to ground current of the power supply to comply with U.L. Standards while also providing a stable DC output voltage which is virtually unaffected by changes in magnitude of the AC supply voltage and by changes in temperature, so that the power supply is suitable for providing power to, for example, a motion sensor switch.